Shoe soles in which a reinforcement member that is matched to the shape of the arch of the mid sole is provided in the arch portion of the shoe soles are known in the prior art, e.g., shoe soles in which a portion of the mid sole that is not attached to the outer sole does not come into contact with the ground upon landing of the outer sole. Such a reinforcement structure suppresses the deformation of the mid sole, thereby reinforcing the rigidity of the arch portion of the mid sole. Examples of such known structures (the first and second patent documents) are shown in FIGS. 15A and 15B.    First Patent Document: Japanese Laid-Open Patent Publication No. 2003-19004 (FIG. 5)    Second Patent Document: WO2005/037002A1 (Abstract)
FIG. 15A is a side view of a shoe sole disclosed in Japanese Laid-Open Patent Publication No. 2003-19004 (FIG. 5) (laid open on Jan. 21, 2003). In the shoe sole, an arch 102 is formed in a bottom portion of the arch portion of a mid sole 101. A first reinforcement member 103 is attached to the lower surface of the arch 102, and a second reinforcement member 104 is provided below the first reinforcement member 103.
When a load is applied on the shoe sole of FIG. 15A, the wearer will feel an upthrust on the arch of the foot.
FIG. 15B is a cross-sectional view showing a shoe sole disclosed in WO2005/037002 A1 (laid open on Apr. 28, 2005). Referring to this figure, a hole 203 is provided in the lower surface of a first arch 201, and a protrusion 204 that can fit in the hole 203 is provided on the upper surface of a second arch 202.
These pieces of prior art disclose providing a plurality of members vertically spaced apart from each other in the middle foot portion of the shoe sole, in view of the bending or twisting load to be applied to the middle foot portion of the foot.